1. Field of the Invention
This invention relates to manufacturing of a hollow plastic product and in particular to a method and apparatus for manufacturing a hollow plastic product having an irregular structure, such as a projection which projects outwardly from the outer surface of the product or change in composition. More specifically, the present invention relates to a blow molding technique for fabricating a hollow plastic product having a desired shape which cannot be manufactured by conventional blow molding technology.
2. Description of the Prior Art
If it is desired to manufacture a hollow plastic product 2 having a projection 1 which projects outwardly from the outer surface of a main body 2' in accordance with the typical prior art technique, the main body 2' is first fabricated by blow molding and then the projection, which has been previously fabricated by any appropriate method, such as injection molding, is fixedly attached to the main body 2'. Such an approach is not advantageous because it requires multiple separate steps to carry out in manufacture and the connection between the main body 2' and the later-attached projection 1 is not reliable and low in strength. On the other hand, it is true that the projection 1 may be manufactured together with the main body 2' at the same time by blow molding; in this case, however, a significantly large flash is necessarily formed, and, thus it is expensive and time consuming to remove such a large flash, not to mention the significant waste of material.
Furthermore, it is sometimes desired that a hollow plastic product differs in composition depending on the location, thereby providing different properties. For example, in a hollow plastic tube it is often desired to have a relatively hard section to be fixedly attached to a metal fitting or the like and another section which is relatively soft for providing an easy bending characteristic. In such a case, a typical prior art approach would be to first form different sections separately and then put them together to form a final product. This approach is disadvantageous because such a multistep process is time consuming and expensive and the integrity of the final product is relatively poor.